paulp

package p

/** Super basic map/flatMap fusion ADT.
 */
sealed trait View[A] {
  def map[B](f: A => B): View[B]           = CView(this, Mapped(f))
  def flatMap[B](f: A => View[B]): View[B] = CView(this, FlatMap(f))

  def foreach(f: A => Unit): Unit = this match {
    case IdView(xs)            => xs foreach f

pathikrit

val n = 9
val s = Math.sqrt(n).toInt
type Board = IndexedSeq[IndexedSeq[Int]]
 
def solve(board: Board, cell: Int = 0): Option[Board] = (cell%n, cell/n) match {
  case (r, `n`) => Some(board)
  case (r, c) if board(r)(c) > 0 => solve(board, cell + 1)
  case (r, c) =>
    def guess(x: Int) = solve(board.updated(r, board(r).updated(c, x)), cell + 1)  
    val used = board.indices.flatMap(i => Seq(board(r)(i), board(i)(c), board(s*(r/s) + i/s)(s*(c/s) + i%s)))

pthariensflame

The Indexed Privilege Monad in Haskell, Scala, and C#

We've already looked at two different indexed monads in our tour so far, so let's go for a third whose regular counterpart isn't as well known: the privilege monad.

Motivation

The regular privilege monad allows you to express constraints on which operations a given component is allowed to perform. This lets the developers of seperate interacting components be statically assured that other components can't access their private state, and it gives you a compile-time guarantee that any code that doesn't have appropriate permissions cannot do things that would require those permissions. Unfortunately, you cannot easily, and sometimes cannot at all, build code in the privilege monad that gains or loses permissions as the code runs; in other words, you cannot (in general) raise or lower your own privilege level, not even when it really should be a

pthariensflame

The Indexed Continuation Monad in Haskell, Scala, and C#

The indexed state monad is not the only indexed monad out there; it's not even the only useful one. In this tutorial, we will explore another indexed monad, this time one that encapsulates the full power of delimited continuations: the indexed continuation monad.

Motivation

The relationship between the indexed and regular state monads holds true as well for the indexed and regular continuation monads, but while the indexed state monad allows us to keep a state while changing its type in a type-safe way, the indexed continuation monad allows us to manipulate delimited continuations while the return type of the continuation block changes arbitrarily. This, unlike the regular continuation monad, allows us the full power of delimited continuations in a dynamic language like Scheme while still remaining completely statically typed.

pthariensflame

The Indexed State Monad in Haskell, Scala, and C#

Have you ever had to write code that made a complex series of succesive modifications to a single piece of mutable state? (Almost certainly yes.)

Did you ever wish you could make the compiler tell you if a particular operation on the state was illegal at a given point in the modifications? (If you're a fan of static typing, probably yes.)

If that's the case, the indexed state monad can help!

Motivation

ngocdaothanh

package recfun

import scala.collection.mutable.ListBuffer
import common._

/** https://class.coursera.org/progfun-2012-001/assignment/view?assignment_id=4 */
object Main {
  def main(args: Array[String]) {
    println("Pascal's Triangle")
    for (row <- 0 to 10) {

retronym

class A
class A2 extends A
class B

trait M[X]

//
// Upper Type Bound
//
def upperTypeBound[AA <: A](x: AA): A = x